1. Field of the Invention
The present invention relates to light guide lenses and light emitting diode (LED) package structures, and more particularly, to a light guide lens and a light emitting diode (LED) package structure for turning incident LED light coming in all directions into laterally exiting light.
2. Description of the Prior Art
Backlight modules of conventional liquid crystal displays use mostly cold cathode fluorescent lamps (CCFLs) as light sources. Cold cathode fluorescent lamps, however, contain mercury, consume too much power and have short lifetime. Hence, the current trend is toward replacing cold cathode fluorescent lamps by light emitting diodes (hereinafter referred to as LEDs) when designing the light sources of backlight modules. LEDs are durable and contamination-free. Red, green, blue monochromatic LEDs have advantages, such as a single wavelength and a wide color gamut, and thus resultant liquid crystal display (LCD) images are brightly colored and delicately outlined. However, an LED is a point source and thereby is unlikely to provide uniform illumination for a light emission surface of a backlight module, and in consequence granular hot spots are visible in front of the backlight module.
From an optical point of view, uniform illumination emerges as light travels further away from a point source. Inasmuch as the appeal of LCDs lies in the compactness thereof, an LCD whose uniform illumination-providing surface source is fabricated at the cost of a thickened backlight module is necessarily bulky and thereby is hardly in line with the trend of LCD products. In order to enhance the practicability of LED backlight modules, U.S. Pat. Nos. 6,607,286 and 6,679,621 disclose a lens structure configured for lateral emission of LED light with a view to providing uniform illumination.
Referring to FIG. 1 which is a schematic view of a lens cap disclosed in U.S. Pat. No. 6,607,286, the lens cap 1 mounted on a light emitting diode 2 comprises a concave inner surface 10 and a serrate outer surface 11. The light emitting diode 2 is covered by the inner surface 10 of the lens cap 1. Light rays emitted by the light emitting diode 2 penetrate the inner surface 10 before being refracted by the outer surface 11 and then exiting laterally; among the light rays, oblique rays have to undergo total internal reflection once before being refracted and exiting.
The serrate contour of the outer surface 11 is designed in accordance with calculations relating to the incoming directions of light rays from the light emitting diode 2 such that eventually the light rays laterally exit the outer surface 11. However, the serrate outer surface 11 can only direct incoming light rays at a rough target, and thus it is not true that light rays coming in any direction necessarily exit laterally. In particular, light rays emitted in the direction of the top of the light emitting diode 2 inevitably bring about a hot spot. Hence, in practice, the lens cap 1 is superimposed by a transparent panel having an affixed reflective sheet configured to hide the hot spot from view, thus resulting in a complicated, high-cost assembly process.
As mentioned above, incident light undergoes refraction or total internal reflection once and then goes through refraction once before exiting laterally. Where light travels from an optically denser medium to a less dense medium, loss of light energy increases with an incident angle. LCD illumination is affected as a result of the loss of light energy when light emitted by the light emitting diode 2 is refracted by the lens cap 1.
The serrate structure of the outer surface 11 of the lens cap 1 is so complicated that it has to be fabricated by means of a combination of dies, thus increasing production costs.
In addition, U.S. Pat. No. 6,679,621 has the aforesaid drawbacks. Accordingly, an issue that needs an urgent solution is related to endeavors to overcome the aforesaid drawbacks.